The present invention relates to application of wear-resistant material to a substrate by diffusion bonding, and more particularly to a method of brazing a wear resistant alloy to a turbine blade, and turbine blades having a wear-resistant alloy brazed thereto in accordance with the method.
Certain gas turbine blades have shrouds at the outer extremity of the airfoil. The blade shrouds are typically designed with an interlocking feature, usually in the form of a notch, which allows each blade to be interlocked at its shroud with an adjacent neighbor blade when such blades are installed about the circumference of a turbine disk. This interlocking feature assists in preventing the airfoils from vibrating, thereby reducing the stresses imparted on the blades during operation.
Unfortunately, turbine blades are typically made of nickel-based superalloys or other high temperature superalloys designed to retain high strength at high temperature, and the shroud material of the blade and the interlocking "notch" is not of a sufficient hardness to withstand wear stresses and rubbing which occur during start-up and shut-down of a turbine engine as the blades twist to an "interlocked" and "non-interlocked" position, respectively. Due to the relatively low Rockwell hardness of the shroud materials, the interlocks wear and cause gaps to open in the shrouds, thereby allowing the airfoils to twist and further deform, and even to possibly vibrate during operation which is extremely undesirable as such imparts additional higher stresses on the blades which can quickly lead to blade breakage and consequent failure of the turbine.
As a means of increasing the hardness of the blade shrouds at the interlock interface between adjacent blades, it is known to machine the shroud interlocks under-dimension and apply, by means of a flame-spray operation, a high hardness material, namely a chrome-carbide material to the interlock surface. The high hardness chrome-carbide material is applied using such flame spray operation to a thickness sufficient to bring the shroud interlock face up to the designed dimensional tolerances. Disadvantageously, however, the application of a harder material to the interlock area of the turbine blades (the so-called "Z-notch" area of the blade shroud) in the aforesaid manner produces a hardfacing that is porous, and has low bond adhesion to the nickel-based superalloy which the shroud is comprised of. The porosity makes it difficult to determine the exact dimensions of the thickness of the hardface material, thereby creating problems in interlocking due to the build-up of tolerances at the interlock interface around the periphery of a turbine disk. Moreover, the low adhesion causes the hardfacing to fall off during turbine operation thus re-introducing the original problem of worn, undersize interlocks and undesired stressing of non-interlocked blades.
As an alternative, welding of hardface material to the shroud is sometimes carried out. Again, in this method, the interlocks are machined under dimension, and the hardfacing of a greater-than-desired thickness is applied to the interlock `Z-notch` by welding and thereafter machined so as to produce an interlock Z-notch of the desired dimensional tolerances. Undesirably, however, welding necessarily entails melting of both the hardfacing material and the shroud substrate, which causes an undesirable mixing of the two materials and not only a resultant diminution in the hardness of the hardfacing material but also a weakening of the superalloy characteristics of the shroud material substrate in the vicinity of the weld. Moreover, welding frequently causes cracking during manufacture due to the thermally induced stress gradients, and further often produces non-uniform hardness and porosity in the hardfacing material. Creation of cracks in highly-stressed materials is extremely undesirable. Accordingly, a need exists for a method to bond hardface materials to blades which is not porous and to which dimensional tolerances can easily be met, which has high bond strength suitable for bonding highly-stressed components.